The biomechanical properties of soft biological tissue involve its elasticity, dynamical stiffness, creepability, and mechanical stress relaxation time.
In evidence-based medicine, both the parameters characterising the stress of superficial soft biological tissues, for example of skeletal muscle, and its biomechanical properties are used as a supplementary source of information. The said parameters allow specialists to quantitatively determine the extent of pathological processes, and the efficiency of various massage techniques, physiotherapeutic procedures, medication and training programmes, as well as ascertaining the tone of tissues during an operation, and fixing the time of death in forensics.
Until now, many attempts have been made to measure the stress (tone) of soft biological tissues by various methods, but neither has such a device been invented nor such a method found yet that would measure all the variables characterising the abovementioned parameters in a way that is universal and realisable/applicable in daily clinical practice in real time.
Tone is defined as the mechanical stress of skeletal muscle with no voluntary contraction of the muscle. If we multiply the numerical value of the skeletal muscle stress by its cross-section area, we get the value of the force by which the tendon of skeletal muscle is pulling the periosteum of the bone.
There are three types of tone:                1) The passive resting tone—a state of skeletal muscle with no contraction in the muscle when the muscle is not balancing force torques on the observed joint axis caused by the force of gravity with its mechanical tension. There is no electomyographic (EMG) signal.        2) The resting tone (relaxation) —a state of mechanical stress (or tension) of skeletal muscle without voluntary contraction with EMG activity due to, for instance, an emotional or pathological condition. Such a state is more variable than the passive resting tone. The muscle force torques in antagonist muscles are balanced.        3) The postural tone is a state of skeletal muscle in which the muscle is balancing the force torques of body segments caused by the force of gravity in order to maintain the equilibrium position. When keeping the position, the muscle tension and stiffness are changing persistently, the variability of which is several times greater than in passive relaxed tone. The state of mechanical tension and stiffness level are also significantly higher.        
The tone of the skeletal muscle cannot be decreased at will. The level of the tone depends on intramuscular pressure—the higher the intramuscular pressure, the greater the mechanical tensile stress in the muscle (Vain A. 2006 The Phenomenon of Mechanical Stress Transmission in Skeletal Muscles. Acta Academiae Olympiquae Estoniae, Vol 14, No. 1/2 pp. 38-48). If the intramuscular pressure is high, the outflow of venous blood from the muscle will slow down because the veins have no substantial internal blood pressure and when the intramuscle pressure rises, then the veins' cross-section area will decrease. In the case of passive rest, this causes the situation that skeletal muscles' ability to work is restored slowly. Additionally, the ergonomic efficiency of muscle activity in performing movements will decrease since the moment of force caused by antagonist muscles for turning the part of the body on the axis of the joint increases on account of the work needed to stretch the antagonist muscles. The amount of work A done when stretching the antagonist muscles can be calculated by the following formula:A=Fresistance*s(J)whereFresistance—resistant force (N),s—extent of stretch (m),whereasFresistance=2*v*f*D*m(N),where                v—speed of stretching (m/s),        f—muscle's natural oscillation frequency (Hz),        D—logarithmic decrement of a muscle's natural oscillation,        m—mass of the muscle being stretched (kg).        
It is technically complicated to measure skeletal muscle's state of mechanical stress. However, there has been revealed a functional connection between a material's natural oscillation frequency and its mechanical stress, which in the case of short-term measurements makes it possible to characterise the mechanical state of skeletal muscle.
The logarithmic decrement of a muscle's natural oscillation shows how much mechanical energy dissipates during one period of the muscle's natural oscillation. Hence, the elasticity of skeletal muscle (one of the biomechanical qualities of the muscle) can be characterised via the logarithmic decrement of the muscle's natural oscillation. Elasticity of soft biological tissue means its ability to restore its former shape after the deforming force is removed. The opposite term to elasticity is plasticity. If an elastic body changes its shape as a result of an impulse transmitted by external forces, then simultaneously mechanical energy of elasticity is stored in the morphological structures of skeletal muscle which possess elasticity properties. When the impulse from the deforming force ends, then the stored mechanical energy will restore the body's initial shape at a velocity that accords to the value of the logarithmic decrement—very quickly if the value approaches zero, and more slowly if the value is higher. Hence, in a device built to register the parameter characterising elasticity, the effect of oscillation damping must be brought to a minimum.
In a working muscle, contraction and relaxation alternate. The duration of each may vary. Sometimes it may last only a split of a second. If the relaxation period is short and the muscle's logarithmic decrement is big, then the initial shape of skeletal muscle fails to be completely restored, the muscle's internal pressure falls insufficiently and, as a result, the outflow of venous blood from the muscle is slowed down. The time taken for the muscle's work capacity to be restored increases, its fatigue also increases, and the danger of a muscle overload trauma becomes a reality.
Stiffness is a biomechanical property of skeletal muscle which consists in its resistance to any force changing its shape. The property inversely proportional to stiffness is compliance. The unit of measurement of stiffness is N/m. How economical and how accurately co-ordinated a person's movements are depends on the stiffness of his/her skeletal muscles. Creepability is a biomechanical property of soft biological tissue to deform permanently under constant stress. The creepability property of liquids has been quantitatively measured (U.S. Pat. No. 4,534,211, Molina O. G. 1985).
The creepability property of soft biological tissue might be characterised, for example, by the Deborah number De. The Deborah number is a quantity whose dimension is 1; this number is used to characterise the viscoelasticity of tissues (or creepability of materials). The latter is expressed as the ratio of relaxation time, tmaterial representing the intrinsic properties of tissue, and the characteristic time scale of an experiment, or deformation time, tprocess:De=tprocesstmaterial.
The relaxation property of skeletal muscle tissue is defined as the tissue's ability to relieve itself of mechanical stress in the case of constant length.
The viscoelastic properties of skeletal muscle tissue are characterised by creepability and relaxation (Fung Y. C. 1981 Biomechanics. Mechanical Properties of Living Tissues p. 41).
Various attempts have been made to measure the state of mechanical stress and biomechanical properties of soft biological tissues in vivo. As a result, humanity knows a host of instruments for measuring mechanical stress and stiffness, but no ways have been invented as yet to express creepability and relaxation time of mechanical stress in numerical terms. No such devices or methods are known that would simultaneously measure muscle tone and all the four abovementioned biomechanical properties in real time.
The principal problem is how to evaluate the state of a person's skeletal muscles on the basis of measurement data, while the parameters characterising this state are constantly changing due to their involvement in biological processes. Therefore, it is insufficient to represent the state of soft biological tissue by one parameter only, which reflects the level of measurable quantities; considering the aspect of diagnostic information, it is relevant that a characteristic describing the variation of levels be added. For assessment of variation, it is important that the reading of the measuring device be repeated in short-term measuring scales (e.g. measuring after every 1 second). In this case, measuring should be carried out and monitored by measuring software (firmware), in order to collect in a short term a sufficient amount of measurement data for statistical assessment. No such methods of measurement are known as yet in the diagnostics of soft biological tissues.
Indeed, both methods and devices are available for numerical characterisation of biological tissues' viscoelasticity (e.g., WO2007144520 Method of measuring Viscoelastic Properties of Biological Tissue Employing an Ultrasonic Transducer, EchoSens S.A., 2006), but neither methods nor devices have been disclosed to date that would separately characterise creepability and relaxation properties of soft biological tissues.
None of the earlier solutions allow measurement to be repeated in a short term because the impact on soft biological tissue tends to change the measurable quantities, the character of the measurements is not standardised, and the impact does not end with a quick release.
Among the known solutions, the method closest to the present invention is the myometer, a device and method for recording of mechanical oscillations in soft biological tissues (EE03374B1, Vain A. 2001). The essence of the myometer lies in causing a short-term effect on soft biological tissue by giving it a mechanical impulse and subsequently recording the tissue's mechanical response by means of an electromechanical sensor (acceleration sensor).
One drawback of this solution of the closest prior art is that while the obtained acceleration graph enables calculation of the tissue's natural oscillation frequency, indicating its state of stress as well as the logarithmic decrement characterising its elasticity and dynamic stiffness, it does not make it possible to determine the parameters describing creepability and relaxation time of mechanical stress. Secondly, the parameters characterising the tissue's state of mechanical stress, elasticity and stiffness are calculated at different moments of the oscillation, which yields varying results since the mass participating in the oscillation process decreases constantly due to dissipation of mechanical energy in the case of damped oscillation.
Resulting from the construction of said device (inclusion of a lever), the impulse may be followed by resonant oscillations of the parts exerting impact. If the size of the device is reduced, then the shoulder of the lever will become so short that it will cause a ‘scraping’ impact, which may yield incorrect results as the direction of the tissue's deformation changes during stimulation. Another shortcoming is the constructional solution of the above prior art device, in which bending of the signal cable attached to the acceleration sensor during oscillation will bring about dissipation of the energy of impact.
A shortcoming of the cited prior art device is also the feature that the construction of the measuring apparatus involves rotating details, which need fine tuning to minimise resistance caused by mechanical friction. But the greater the resistance, the less sensitive the device.
An additional drawback of the said closest prior art device is that in such cases when the direction of the testing end with respect to the Earth's gravitational field is changed, the pre-pressure exerted by the mass of the testing end on the superficial tissues covering the muscle will decrease. However, preservation of constant pre-pressure is necessary for delivering the impact energy to the muscle and thereby making it oscillate. If the pre-pressure decreases, the role of superficial tissues grows both in recording the muscle's natural oscillation frequency and in the resulting measurements.
Thus, there exists a need for such a device and method that would allow us to measure in real time, simultaneously, quickly and accurately soft biological tissue's mechanical state of stress and parameters characterising its four biomechanical properties: elasticity, dynamic stiffness, creepability and mechanical stress relaxation time, and achieve, irrespectively of the position of the device in the gravitation field, high sensivity of the device as well as repeatability and reliability of the results.